An example analog signal transfer system is an audio signal transfer system such as a wireless microphone system. In a wireless microphone system, companding is carried out to reduce inflow of noise from a noise source through a wireless transfer path.
Companding is a portmanteau word of “compressing” and “expanding”. In detail, companding represents compression and expansion of the dynamic range of electric signals.
A circuit or element compressing signals is called a compressor. A circuit or element expanding signals is called an expander. A combination of a compressor and an expander is called a compander. Companding can be carried out to transfer signals having wide dynamic ranges through an apparatus or line having only a narrow dynamic range for transferring and reduce noise in received signals.
A transfer path for electric signals is exposed, for example, to interference radio waves from an adjacent signal source and to noise emitted from a circuit of a transmitter or receiver in a signal transfer system. A transfer path for signals has a limit of its dynamic range and is therefore susceptible to noise flowing thereinto. Companding is effective in a reduction in noise in such a signal transfer path. For example, if an encoder in a transmitter compresses a dynamic range into ½ through a logarithmic compression circuit having a ratio of 1:2, a decoder having a ratio of 2:1 in a receiver conversely expands the dynamic range into double.
Companding having a high expansion ratio corresponding to a high compression ratio leads to a variation in noise level in synchronization with a variation in signal level. In a wireless microphone system, this variation in noise level is called breathing. A variation in the level of compressed and expanded signals leads to a continuous variation in noise level. This amplitude-modulates the noise. Even amplitude-modulated noise having a low level is audible much more uncomfortably than continuous noise having a constant level. This sounds like breathing in background of audio signals such as music. This sound like breathing is called “breathing” or “breathing noise”. Dynamic noise having a correlation with audio signals such as breathing noise is significantly uncomfortable for a listener and causes significant deterioration of the sound quality.
In a well-known wireless audio noise reduction system including a compander disclosed in Patent Publication WO2003/030387, a variable compressor compresses signals exceeding a first predetermined input threshold value at a higher compression ratio than 1:1, and a variable expander expands signals exceeding a second predetermined input threshold value at a higher expansion ratio than 1:1.
The invention disclosed in Patent Publication WO2003/030387 switches the compression ratio and the expansion ratio each between two levels depending on whether a signal level exceeds the predetermined threshold value, and thereby reduces breathing noise. Unfortunately, this invention causes noise or abnormal sound at a bent point as a boundary at which the compression ratio and the expansion ratio are each switched between the two levels.